Week 3

Question 1

How is data represented in a relational data model

Answer 1

A collection of tables / a collection of relations

tables are called relations

Question 2

What data is stored in rows

Answer 2

entities and relationships

Question 3

What is in the column header

Answer 3

Attributes / attribute name

Question 4

A row is also known as a …

Answer 4

tuple

Question 5

A row contains an instance of what ever the relation name is . eg for a STUDENT (relation name) each row represents a specific student instance

Answer 5

bkjbjk

Question 6

what does a key do in relational data model

Answer 6

Uniquley identifies a specific row ( and hence a specific instance as row = specific instance)

Question 7

what is a domain

Answer 7

set of atomic (non -divisible) values allowed for an attribute

specifies the data type allowed for an attribute

Question 8

what are surrogate / artificial keys

Answer 8

a unique identifier assigned to each record in a table,

eg I can increment studentID from by 1 each time to identify a student uniquely and that is a surrogate key

Question 9

state

Answer 9

The content / data in the table at a particlular time

subset of the cartesian product of values in domain

Question 10

tuples (rows) in a relational data model are…

Answer 10

unordered

For ease in the course , we will assume attributes are ordered as well

Question 11

what does t[Ai]

Answer 11

for an arbituary tuple (row) t , the attribute denoted by the value of i

eg t[A2] the second attribute at the required tuple

Question 12

What are the 3 main types of constraints in the relational data model

Answer 12

.Key constraints
.Entity integrity constraints
.Referential integrity constraints

implicit constraint is the domain constraint:
values in an attribute must be from the domain of that attribute

Question 13

superkey info

Answer 13

a superkey is a set of one or more attributes (columns) that, taken together, can uniquely identify a tuple (row) in a table.

A key, on the other hand, is a minimal superkey, meaning it is a superkey with the fewest possible attributes. If removing any attribute from a key would cause it to lose its uniqueness, then that key is considered minimal.

Question 14

Why is {serial no , Make} a superkey for a car

Answer 14

. At least one of the attributes can uniquely identify a car instance (tuple) [ serial no uniquely identifies}

However if you remove the make attribute it is still a superkey - Not minimal superkey - Therefore not a key

Question 15

foreign key

Answer 15

. A foreign key is an attribute (or a set of attributes) in one relation (table) that establishes a link or reference to the primary key in another relation.

eg serial number ( primary key) for a CAR relation may link to a DRIVER relation

general rule of thumb - use the smallest sized key for primary key

Question 16

What is a relational database schema

Answer 16

A set of the relation schemas

S = {R1,R2…Rn}
where R1,R2,…,Rn are names of the relations

Question 17

Entity integrity

Answer 17

Ensuring values for primary keys are not null

If primary key contains several attributes, none of them can be null

Question 18

Referential key integrity

Answer 18

The foreign key in one relation has to have the same domain as the primary key it refers to in another relation

Neso Academy Relational model constrains

A Value that occurs in the foreign key has to be present in the primary key

t[Fk] = t[Pk]

Question 19

key constraint

Answer 19

No two tuples can have same value for a superkey / key( as a key is just a minimal superkey) since they uniquely indetify a tuple
(uniqueness is removed if more than 1 tuple has same value as you wont be able to identify)

. key must be a minimal superkey ( if you take an attribute away you can no longer uniquely identify)

Question 20

what is a semantic integrity constraint

Answer 20

constraint specific to the database your building

eg: grades < = 100 (and greater than or equal to 0 )